The present invention relates to a tunable capacitor and in particular to a steplessly tunable micromechanic high-frequency capacitor.
Tunable capacitors of a high quality factor Q are required, for example, in tunable oscillators, so-called VCOs=voltage control oscillators, and in high-frequency and microwave circuits. Capacitance diodes, also referred to as varactor diodes, are responsible for changing the capacitance. Application of a reverse-biased voltage changes the space charge distribution in the barrier layer of these diodes, and thus their capacitance. In this way, the capacitance decreases as the reverse-biased voltage increases. Typically a change in capacitance of 10 pF to 2 pF is achieved with a change in the reverse-biased voltage from 1 V to 10 V. These diodes, however, have drawbacks that become apparent in particular in microwave oscillators. Thus, for example, the high series resistance leads to low resonance quality factors Q, and noise increases with higher frequencies. A further drawback is that the capacitance""s swing of the diodes is limited to about 2 pF at the bottom end.
In the high-frequency or microwave range and in designing appropriate components, it is further always desirable to have adjustable or variable components such as, for example, adjustable capacitors, adjustable resistors or adjustable inductive devices. Such components provide a developer in the high-frequency and microwave ranges with high flexibility in relation to the design of high-frequency and microwave circuits. In re-configurable radio systems of the future, such as, for example, software-based radio systems, these components are referred to as enabling components/technology. This is due to the fact that software-based radio systems require a programmable high-frequency receiver unit (HF receiver unit) which, in addition to other requirements placed upon the high-frequency components, supports at least different frequency bands. In conventional components technology, in particular when mobile radio applications are considered, this represents a big challenge since power consumption, space requirements, weight, as well as electrical behavior, represent very demanding challenges.
The well-known varactor diode described above is, for example, one type of an adjustable capacitor. In addition to the drawbacks mentioned above, a further drawback is that the capacitance of the varactor diode changes in a non-linear manner with the control voltage applied, and that the Q factor of such a varactor diode is generally very low, at about 40. These features limit the range of employment of varactor diodes to applications where the requirements with regard to linearity and the Q factor (the quality) are not demanding. Typical examples of the use of varactor diodes are the above-mentioned voltage-controlled oscillators (VCOs) as well as some variable filters.
In view of these drawbacks, manifold research activities have recently been developed worldwide so as to find solutions for the above adjustable components.
Several concepts for adjustable capacitors have been suggested. These concepts are based on so-called MEMS technology, MEMS standing for xe2x80x9cmicro-electro-mechanical systemsxe2x80x9d. MEMS technology is an up-and-coming technology that has attracted more and more attention in the last few years. The adjustable MEMS capacitor can be seen as a counterpart of the semiconductor varactor diode.
There are numerous requirements placed upon an MEMS capacitor for use in programmable high-frequency circuits. Ideally, an MEMS capacitor should first of all be a linear device. This means that the capacitance of the MEMS capacitor does not change with the high-frequency signal when a fixed control voltage or a fixed control current is applied. Secondly, the MEMS capacitor should have a high Q factor (high quality), i.e. low losses. Thirdly, an MEMS capacitor should have a broad variability range. The tuning range typically required for mobile phones is from 0.5 pF to 20 pF. In addition, the MEMS capacitor should not consume any power and be readily integratable with other circuits and circuit components.
No MEMS capacitor is currently known which might be available from any manufacturer. However, there are several concepts, such as, for example, the approach described in the essay by T. T. C. Nguyen, xe2x80x9cMicromachining Technologies for Miniaturized Communication Devicesxe2x80x9d, Proc. SPIE Conference on Micromachined Devices and Components IV, SPIE volume 3514, September 1998, pp. 24-37. This essay describes a continuously variable capacitance which is actuated in an electrostatical manner and wherein a movable electrode consists of a metallic membrane suspended on a spring beam.
Generally speaking, an MEMS capacitor principally consists of two metal plates, the distance and/or area (overlap area) is controlled by actuators. These actuators are controlled by a voltage in the case of electrostatic actuators, or by a current in the case of thermal actuators.
An example of an MEMS capacitor known from the prior art is described in more detail with reference to FIG. 4. FIG. 4A shows a plan view representation of a known MEMS capacitor, and FIG. 4B shows a cross-sectional representation of the capacitor shown in FIG. 4A.
The MEMS capacitor 400 is formed by a fixed electrode 402 and a movable electrode 404, those electrodes 402 and 404 having essentially the same surface area A. The movable electrode 404 is arranged to substantially overlap the fixed electrode 402. The fixed electrode 402 and the movable electrode 404 are spaced apart by a distance x. The movable electrode 404 is supported by a first suspension 406 and a second suspension 408, the suspensions 406 and 408 being designed so as to move the movable electrode 404 such that the distance x between the electrodes 402 and 404 changes.
The first suspension 406 is fastened on a first fastening element 410, and the second suspension 408 is fastened on a second fastening element 412. These elements 410 and 412 in turn are fastened on surface areas (not shown), such as on a substrate or the like. The first suspension 406 includes a first portion 406a, a second portion 406b and a third portion 406c. The second portion 406b includes an actuator connected between the first portion 406a and the third portion 406c. The first portion 406a is further connected to the first fastening element 410, and the third portion 406c is connected to movable electrode 404. Similarly, the second suspension 408 includes a first portion 408a, a second portion 408b as well as a third portion 408c; the portion 408b also including an actuator. The first portion 408a is further connected to the second fastening element 412, and the third portion 408cis further connected to the movable electrode 404. As can be seen in FIG. 4A, the portions 406b and 408b are arranged in an inclined manner so that the electrode 404 is arranged to be spaced apart from the electrode 402.
The electrode 402 includes a high-frequency input connection 414, as is illustrated by the arrow 416. The second fastening element 412 includes a high-frequency output connection 418, as is illustrated by the arrow 420. The path of a high-frequency signal through the capacitor 400 thus extends from the connection 414 via the electrode 402 to the electrode 404 to the output 418. With regard to the suspensions 406 and 408 it is to be noted that they are not insulated for the high-frequency signal path.
For driving the actuators 406b and 408b, a control connection 422 is provided which is connected to the second fastening element 412 via a line 424. Application of an appropriate control signal to the connection 422 leads to an actuation of the actuators 406b and 408b, whereby a displacement of the movable electrode 404 is caused, so that the distance x between the movable electrode 404 and the fixed electrode 402 changes.
The capacitance C of the capacitor 400 is calculated as follows   C  =            ϵ      r        ·          ϵ      0        ·          A      x      
wherein:
∈r=relative permittivity for the dielectric arranged between the plates,
∈0=electric field constant,
A=surface area of the capacitor plates, and
x=distance of the capacitor plates.
FIG. 4B depicts a cross-sectional representation of the capacitor shown in FIG. 4A, and the same reference numerals are used for corresponding elements.
Due to the use of the two electrode plates 402 and 404 it is necessary to employ a part of the suspensions 406, 408, and in particular of the actuators 406b and 408b, as a part of the high-frequency signal path. The capacitance of the MEMS capacitor 400 is controllable (variable or adjustable) by adjusting the voltage or current applied to the connection 422. Various materials, such as a silicon, GaAs (gallium arsenide), a ceramic substrate or the like may be used as the carrier substrate (not shown in FIG. 4).
The type of an MEMS capacitor shown in FIG. 4, however, has similar drawbacks as a varactor diode. Due to the fact that the distance x and/or the surface area A between the two electrode plates 402 and 404 does not necessarily change in a linear manner with a control voltage or control current applied, the capacitance changes just as little in a linear manner with the control voltage or control current applied. Since both the control signal (voltage and/or current) and the high-frequency signal occur at the same connection of an MEMS capacitor, such an MEMS capacitor will be a non-linear device, in particular with large high-frequency signals. Just as with a varactor diode, the capacitance may change with the high-frequency signal even when the control signal (voltage and/or current) is fixed. No publications are known in the prior art that address the linearity problem in such MEMS capacitors.
Even though many publications intensely concentrate on the quality (Q factor) and the variability range, it shall be emphasized that with the known capacitors, either the reported Q factor is very low (e.g. much lower than 100 at frequencies of about 1.5 GHz) or the variability range achieved is very limited (e.g. is only from 0.5 pF to 3.5 pF).
It is the object of the present invention to provide an improved tunable high-frequency capacitor which is a linear device having a high quality factor and a large variability range.
The present invention provides a tunable high-frequency capacitor having a first fixed electrode having a high-frequency input connection, a second fixed electrode having a high-frequency output connection, a movable electrode mounted facing and spaced apart from the first and second fixed electrodes, and a suspension which supports the movable electrode and causes a displacement of the movable electrode in relation to the first and second fixed electrodes depending on a control signal applicable to the suspension.
The present invention is based on a novel concept for an adjustable MEMS capacitor by means of which the drawbacks of varactor diodes can be overcome, so that new adjustable MEMS capacitor devices may be produced which may be used for development of different programmable, both linear and non-linear, high-frequency circuits (LNA, mixers, VCO, PA, filters, etc.), e.g. in re-configurable radio systems, including mobile radio devices etc.
In accordance with an embodiment of the present invention, the suspension includes an actuator which causes the displacement of the movable electrode depending on the control signal applied, and the high-frequency signal path from the high-frequency input connection via the movable electrode to the high-frequency output connection being insulated from the suspension.
In accordance with a preferred embodiment, the capacitor includes a frame structure which supports the movable electrode and on which the suspension is fastened. In addition, an insulating membrane may be provided which is arranged on a surface of the movable electrode; this surface facing the fixed electrode. The insulating membrane may also be supported within the frame structure, in which case the movable electrode is arranged on the insulating membrane.
By means of the suspension and drive by means of a control signal, the present invention enables a movement of the movable electrode in a plane parallel to the fixed electrodes and/or in a plane perpendicular to the fixed electrodes.
In accordance with a preferred embodiment of the present invention the capacitor includes a substrate having a first main surface on which the first and the second fixed electrodes are arranged to be spaced apart. In this case, the suspension is also connected to the first main surface. In accordance with an embodiment, the suspension may be formed by one or several bending beams. Preferably, the suspension is manufactured by means of micromechanical processing.